As the size and scaling of semiconductor device technology is reduced, aspects of device design and fabrication that previously gave rise to only second-order effects in long-channel devices can no longer be ignored. For example, the reduced scaling of channel length and gate oxide thickness in a conventional metal-oxide-semiconductor (MOS) transistor exacerbates problems of polysilicon gate depletion, high gate resistance, high gate tunneling leakage current and device (i.e., boron) penetration into the channel region of the device. As a result, CMOS technology is increasingly replacing silicon dioxide gate dielectrics and polysilicon gate conductors with high dielectric constant (high-k) gate dielectrics in combination with metal gate electrodes formed from a gate stack of polysilicon and one or more metal layers.
Scaling of the gate dielectric is a challenge in improving performance of advanced field effect transistors. A high-k gate dielectric provides a way of scaling down the effective oxide thickness (EOT) of the gate dielectric without an excessive increase in the gate leakage current. However, high-k gate dielectric materials are prone to catalyze oxidation of the underlying substrate because high-k gate dielectric materials react with oxygen that diffuses through the gate electrode or gate spacers. Regrowth of a silicon oxide interface layer between a silicon substrate and the high-k gate dielectric during high-temperature processing steps is a major obstacle to successful effective oxide thickness scaling. Particularly, typical stacks of a high-k gate dielectric and a metal gate are known to be susceptible to a high temperature anneal in an oxygen ambient. Such a high temperature anneal in oxygen ambient results in regrowth of the silicon oxide interface layer and produces instability of the threshold voltage and EOT of field effect transistors.
While high-k dielectrics in conjunction with low sheet resistance metal gate electrodes advantageously exhibit improved transistor performance, the use of new metal layer technologies can create new technical challenges. For example, to optimize drain current and device performance and reduce the voltage threshold Vt, the desired effective work function for negative channel metal-oxide-semiconductor (NMOS) and positive channel metal-oxide-semiconductor (PMOS)) gate electrodes must be near the conduction (or valence) band edge of silicon, meaning that the metals used in NMOS transistors should have effective work functions near 4.1 eV and metals used in PMOS transistors should have effective work functions near 5.2 eV. Since it is difficult to find a material that can have its work function adjusted once it is deposited, approaches for obtaining differentiated work functions have involved forming separate gate electrode layers, such as by removing a deposited first metal gate layer from the gate insulator to deposit a second metal gate layer having a different work function. Such processes can damage the gate insulator layer, leading to high leakage or reliability problems for the finally formed device. Another method for obtaining different work functions involves formation of two gate stacks of unbalanced height which can be a major challenge for the subsequent gate etch process.
Accordingly, a need exists for an improved integration of a metal gate electrode and very thin high-k gate dielectric materials in NMOS and PMOS devices having work functions that are set near the silicon band edges for low voltage thresholds and improved device performance. Further, the integration should reduce or eliminate any interface layer underneath the high-k gate dielectric to enable effective oxide thickness scaling.